1. Field of the Invention
The present invention relates to a fluid operable valve actuator having therein a fluid compressing accumulator cylinder which is continuously charged, whereby venting of control pressure below a given level activates the compression force within the accumulator to shift a valve head in operable association therewith in relation to its seat.
2. Description of the Prior Art
Actuators are utilized to manipulate a valve mechanism within a flow line into open and/or closed position in response to control pressure variation. Normally, these actuators comprise a shaft and a fluid activated mechanism in association therewith which, upon activation thereof by increase in control line pressure, causes longitudinal movement of the shaft to shift the valve in relation to its seat. Venting of control line pressure will cause a subsequent and second longitudinal shifting of the shaft and the valve head to a second position. Such valve systems are frequently utilized in safety systems used in conjunction with the drilling, completion and production of offshore, as well as inland, oil and gas wells. Additionally, such components are utilized in natural gas transmission lines, and the like.
During the completion, testing and/or workover of a subterranean well, it may be necessary to run equipment such as a perforating gun, or the like, on a wire or other line into the well when the well is under pressure. This is achieved by inserting the equipment into a length of production tubing above the christmas tree, the length of tubing being commonly referred to as a "lubricator". The lubricator is isolated from the portion of the well therebelow by a valve or a series of readily accessible hand manipulated valves. In view of the fact that the lubricator assembly must contain the well pressure while the equipment is inserted therethrough for subsequent utilization in the well, it is necessary to control the well pressure below the lubricator assembly during this procedure. To contain the well, in the event of failure of the components of the lubricator, a safety valve mechanism is positioned below the lubricator. This valve should be "fail safe" and should be activatable remotely or automatically upon loss of control.
When a gate valve mechanism is utilized below the lubricator, the valve actuator should be of such design and construction that, upon longitudinal shifting of the stem therein, the gate is permitted to completely close with sufficient force such that a wire line carrying downhole tools may be sheared by the gate upon longitudinal shifting of the shaft within the actuator. Upon detection of pressure leaks within the lubricator, or when control of the well is lost and it begins to prematurely flow, the actuator should have sufficient force to cause the wire line to be sheared when the gate is closed in order to assure that the gate is sealingly interfaced onto its seat to prevent flow therethrough. The force which shears the wire line penetrating through the gate opening should be independent of the well pressure within the valve body.
Normaly, valve actuators depend upon pressure within the valve body operably associated with a compressed spring assembly to shift the bonnet stem longitudinally in order to permit the valve to close. The spring assembly within the actuator is present only for minor friction forces, and fail safe gate valve actuators heretofore made commercially available do not have sufficient force to shear a wire line during the valve closing sequence, in the absence of well pressure in the valve body.
Standard fail safe gate valve actuators are not afforded sufficient force to shear wire inserted through the valve due to physical and technological limitations and sizing requirements for the valves and their associated operating components. A double acting actuator with pressure operable upon either side of a piston element could be utilized in conjunction with spring force to cause manipulation of a shaft within the actuator to, in turn, cause sufficient force to be exerted on the shaft such that longitudinal movement of the shaft shears the wire line inserted through the valve body. It should be noted that this double acting actuator design is not fail safe, because loss of control pressure will not assure closing of the valve head onto its seat. This piston arrangement would require an external charging force, such as a nitrogen accumulator. Consequently, a stock of nitrogen bottles would be required at the well or other commercial site. The nitrogen charged accumulator would force control fluid out of one of the piston chambers, whereby the valve head is shifted to the closed position, Thus, it is clear that longitudinal movement of the stem when control pressure is vented off to close the valve would require two control lines, the first line being in association with a supply port and one piston chamber, and a second control line in a second or boost port communicating with a second or boost piston chamber. In view of the fact that the actuator design must be "fail safe", that is, it must assure closure of the valve when control line pressure is intentionally or unintentionally bled off, the incorporation of two control lines into the design of the actuator renders the design doubly susceptible to failure by leaks and/or breaks in the lines.
As an alternative design, a concentric accumulator exteriorly surrounding the actuator could be utilized as an integrable part of the actuator. This design would require the utilization of nitrogen bottles in a bank for continuous charging of the accumulator. Additionally, the two control lines would, of necessity, still be required to maintain nitrogen charges. Use of plural lines would, in turn, continue to doubly expose the apparatus to the likelihood of nitrogen leaks which are extremely difficult to detect and seal.
The present invention overcomes many of the disadvantages found in prior art commercially available actuators by providing an actuator having an internal cylindrical accumulator with sufficient volume such that the fluid therein may be compressed with sufficient force to assure that the shaft is longitudinally shifted to shear a wire line inserted through the valve body during the valve closing sequence. The accumulator requires no incremental or continuous charging with secondary sources, such as nitrogen; hence, problems associated with secondary source leakage are eliminated. Additionally, in the preferred form, the present invention necessitates usage of only one control line, this advantageous feature being attributable to usage of a check valve within a conduit or passageway extending between the control line and the accumulator chamber and being within the actuator itself. Moreover, the check valve mechanism permits continuous charging of the accumulator chamber so that when supply pressure is vented off, pressure within the accumulator chamber is trapped and compressed fluid therein will drive the piston and its associated shaft in a longitudinal direction to shift the gate or other valve member, for example, to isolate the fluid flow within the valve body. In the present invention, in the event of leakage of O-rings or other sealing elements, the accumulator chamber will continue to be charged with fluid through the control line which will result in a "fail safe" actuator which does not require usage of secondary fluid sources, such as nitrogen bottles or nitrogen caps on the accumulator to drive the fluid under the piston and upwardly to shift the valve.
The presen actuator has a physical design advantage in that it can be manufactured substantially shorter lengthwise than actuators designed to operate within and at similar pressure environments, but responsive to compressibility of spring elements. Although the diameter of the present actuator is somewhat larger than comparative-sized actuators to operative at the same control pressure, the present invention now contains approximately ten times the closing force of spring-fed actuators. For example, a 5,000 pound closing force can be obtained using the continuously fed accumulator chamber of the present actuator. To obtain such a closing force in a spring-fed prior art actuator, ten concentrically mounted 500 pound force springs would have to be utilized. This, in turn, would require an actuator having a housing approximately two times the diameter of the housing of the actuator of the present invention. Additionally, such a spring-fed actuator would be considerably longer (lengthwise) than the present actuator due to space requirements for the action of the spring. Moreover, the absence of a spring element in the present actuator renders it less susceptible to mechanical failure because the risk of spring failure or breakage due to corrosion and/or metal fatigue is eliminated.
The present actuator utilizes a plurality of pressure chamber areas which permits continuous charging of the accumulator because of the variance in the areas of each of the chambers, whereby forces applied through the chambers are selectivly pressurable. Thus, by having independent control of each of the chamber areas, fluid media, control pressure, and operating forces can be independently controlled. This design will result in reduced loads being applied to the bonnet stem.
Another advantage of the present actuator is that the shearing of wire line inserted through the valve is not a function of well pressure through the valve body itself, and there is sufficient pressure acting upon the piston and shaft elements to sufficiently shear a wire line extending through the valve, thus permitting a complete fluid seal between the valve head and its seat, upon closing of the valve